The present invention relates to two-piece fasteners and more particularly to swage type fasteners having a variable clamp capability and to a method for securing workpieces.
The present invention generally relates to two-piece swage type fasteners or lockbolts of the type illustrated in U.S. Pat. No. 3,915,053 to J. Ruhl, issued Oct. 28, 1975, U.S. Pat. No. 2,531,048 to L. Huck, issued Nov. 21, 1950 and to U.S. Pat. No. 4,472,096 to J. Ruhl and R. Dixon, issued on Sept. 18, 1984. All of the above patents can be considered as prior art relative to the present invention.
The fastener of the present invention, as with the prior art noted, includes a pin and a collar adapted to be swaged into lock grooves in the pin in response to a preselected relative axial force applied between the pin and collar.
In many commercial applications, two-piece threaded fasteners are used and are installed in two or more steps. Thus it is common to secure a structure first by attaching the associated members with the fasteners loosely engaged. This facilitates adjustment and/or alignment of the structural members to a final, desired orientation. Next the fasteners are tightened to a preselected final torque. A traditional problem with torque applied threaded fasteners is that the clamp force attained is generally unpredictable due to frictional and other factors i.e. friction between nut and engaged workpiece surface, friction in the engaging threads, etc. Two-piece swage type fasteners, however, are not confronted with such frictional problems and hence can provide more uniformly predictable clamp loads. However, the swage type fasteners have not been capable of providing the initial variable clamp up available with threaded fasteners and still provide a full clamp with only a second, final installation step. Prior threaded swage and/or crimp type fasteners would require extra steps or complex tooling. For example, a threaded crimp type fasteners would not provide the same flexibility without complex installation tools; see for example the U.S. Pat. No. 3,421,562 to J. F. Orloff et al issued Jan. 14, 1969. While the latter fastener would permit an initial variable clamp load similar to a conventional threaded fastener, the final installation would require either two additional steps of final torquing and crimping or a complex tool capable of performing the latter steps in sequence.
In the present invention the lock grooves of the pin are in the form of a helical male thread. The collar is provided with a mating, female thread of a preselected extent such that the initial clamp can be accommodated. However, the female collar thread is selected to be of a limited circumferential extent and shear strength such that the collar can be brought to its final, desired clamp position and swaged to the pin via a conventional installation tool. Thus for the final installation, a conventional pull tool can be used to apply a relative axial force between the pin and the collar. The female collar thread is selected such that, in response to the relative axial force and at a level prior to the initiation of collar deformation or swaging into the lock grooves of the pin, it will shear or deform such that the collar will be free to move axially over the pin and to respond to the installation loads in the same manner as a collar without such limited female thread form. Now the workpieces can be finally clamped together with the same effectiveness as swage type fasteners.
In some applications it is desirable that the fasteners have a high strength, high performance characteristic both in clamp up and in fatigue. In this latter regard then, it is advantageous to utilize the concepts of the invention of the parent C.I.P. application to Dixon (supra).
Thus, in a preferred form of the present invention, the fastener comprises a pin and a collar adapted to be swaged into lock grooves in the pin. The lock grooves in the pin, unlike the lock grooves of the prior art noted, are very shallow and are constructed to have roots of a simulated streamlined shape. As noted the lock grooves are helical and define a desired thread configuration. The shallow grooves and simulated streamlined shape, however, provide a resultant fatigue life which is superior to that of a comparable threaded fastener. At the same time the collar is provided with a predetermined wall thickness, and hence volume, not only to provide an over fill condition of the lock grooves during swaging but also to provide a retained clamp having a magnitude which is a high percentage of the installation load and of the yield strength of the pin in tension across the lock grooves. Thus the collar wall thickness is selected to enhance retained clamp and to provide sufficient hoop stiffness to inhibit spring back of the collar material out of the shallow lock grooves in the pin. The subject fastener will also provide clamp loads which are significantly higher than those achievable with a threaded, solely torqued counterpart because of the absence of the applied torque (and resultant friction) required for the threaded system.
In the present invention, the design failure load can provide a failure in shear across either the shoulders of the pin lock grooves or the shoulders defined by the swaged collar. In other words, a relatively high strength pin is used which will not fail across the engaged grooves at the lower magnitudes of applied tensile loads. Thus, with a given pin, the magnitude of available design tensile load can be increased or decreased simply by varying the length of the collar and hence by varying the number of pin and collar shoulders that are supported the tensile load. This selective variation can occur until the appropriate number of engaged shoulders are provided whereby failure under tensile loading will occur as a tensile failure diametrically across the pin.
In one form of the present invention, the resultant preload applied to the pin is slightly below that load at which yielding would occur in the pin lock grooves. The collar is slightly longer than that required to provide engagement of the appropriate number of pin and collar shoulders to hold that preload; in this case the failure at the design tensile load will be in shear across the collar and/or pin shoulders. By simply increasing the length of the collar the number of engaged shoulders will be increased whereby failure will occur at a higher tensile load in tension diametrically across the pin lock grooves. In either event, the result will be a retained preload of a high magnitude on the fastened joint.
Note that when the high preload levels are considered in conjunction with the advantages of the helical lock grooves which are considerably shallower than the threads or grooves in a comparable conventional, threaded system, the attainable improvement in fatigue and retained stress is even more significant. Some additional gains are secured over an annular groove of a shallow structure with the tensile strength and fatigue life of the pin being somewhat greater since the helical shape will provide a greater diametrical lock groove dimension than the shallow annular groove. Conversely, however, equivalent values could be secured between the helical and annular shallow groove constructions by a slight reduction in the effective cross-sectional area of the threaded construction.
Thus the fastener of the present invention will not only provide significantly improved fatigue characteristics but will also provide a very high clamp load. In one form of the invention the fastener can provide a tensile load carrying capacity up to the tensile strength of the pin across the helical lock grooves which, because of the shallow grooved structure, will be of a maximized magnitude relative to the pin diameter.
At the same time, however, the unique collar and pin combination will permit the desired variable clamp load such that the structural components can be pre-assembled under one load and quickly and efficiently fastened together under high, final clamp loads in a second fastening operation. The latter can be accomplished with a conventional pull tool
It should be noted that the pin can be of a high strength such that there will be no significant yielding in response to the final swage load. The utilization of the desirable shallow groove construction is a significant factor in resisting such yielding. The latter feature is desirable in the present invention where the pin has a helical thread form. Now when the collar is finally swaged into the helical pin grooves, a complementary female thread will be formed in the collar. If the helical thread form of the pin lock grooves are distorted during collar swage, then they will not subsequently function effectively as a threaded fastener with the swaged collar. With the shallow grooved, high strength pin of one form of the present invention such distortion is inhibited. Now the integrity of the thread form is maintained and the swaged collar can be removed by a suitable wrenching tool.
While in one form of the invention the final installation of the fastener can be accomplished with a conventional pull tool, in another form a stump type version of the fastener could be utilized with the final installation effectuated by a squeeze type tool.
Thus it is an object of the present invention to provide a novel two-piece swage type fastener for a fastened joint with a pin having a lock grove section with a helical thread form and with a collar adapted to be swaged into the lock groove section and having a preselected complementary threaded portion allowing a threaded connection whereby a variable clamp can be initially applied to the fastened joint prior to swage.
It is another object of the present invention to provide a novel two-piece swage type fastener of the type noted in the preceding object in which the collar thread is adapted to be sheared or deformed at a preselected load prior to swage but in response to the load applied to the pin and collar to effectuate the initiation of final swaging
It is another object to provide a novel two-piece swage type fastener having a variable clamp characteristic and including a pin having relatively shallow helical lock grooves of a simulated streamlined root configuration whereby fatigue life of the installed fastener is improved.
It is another object of the present invention to provide the two-piece swage type fastener having a variable clamp characteristic and including a pin with shallow grooves of the above described type whereby the final clamp load of the fastened joint can be maximized relative to the installation load and the tensile yield strength of the pin.
It is still another object of the present invention to provide a novel two-piece swage type fastener having a collar including a thread form for facilitating an initial variable clamp and having a preselected wall thickness for providing a magnitude of final retained clamp load which is a high percentage of the installation load applied to the fastened joint.
It is another general object of the present invention to provide a novel two-piece swage type fastener.